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    Subject[PATCH 4.19 271/271] dm bufio: fix deadlock with loop device
    Date
    From: Junxiao Bi <junxiao.bi@oracle.com>

    commit bd293d071ffe65e645b4d8104f9d8fe15ea13862 upstream.

    When thin-volume is built on loop device, if available memory is low,
    the following deadlock can be triggered:

    One process P1 allocates memory with GFP_FS flag, direct alloc fails,
    memory reclaim invokes memory shrinker in dm_bufio, dm_bufio_shrink_scan()
    runs, mutex dm_bufio_client->lock is acquired, then P1 waits for dm_buffer
    IO to complete in __try_evict_buffer().

    But this IO may never complete if issued to an underlying loop device
    that forwards it using direct-IO, which allocates memory using
    GFP_KERNEL (see: do_blockdev_direct_IO()). If allocation fails, memory
    reclaim will invoke memory shrinker in dm_bufio, dm_bufio_shrink_scan()
    will be invoked, and since the mutex is already held by P1 the loop
    thread will hang, and IO will never complete. Resulting in ABBA
    deadlock.

    Cc: stable@vger.kernel.org
    Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/md/dm-bufio.c | 4 +---
    1 file changed, 1 insertion(+), 3 deletions(-)

    --- a/drivers/md/dm-bufio.c
    +++ b/drivers/md/dm-bufio.c
    @@ -1602,9 +1602,7 @@ dm_bufio_shrink_scan(struct shrinker *sh
    unsigned long freed;

    c = container_of(shrink, struct dm_bufio_client, shrinker);
    - if (sc->gfp_mask & __GFP_FS)
    - dm_bufio_lock(c);
    - else if (!dm_bufio_trylock(c))
    + if (!dm_bufio_trylock(c))
    return SHRINK_STOP;

    freed = __scan(c, sc->nr_to_scan, sc->gfp_mask);

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